Horse fly, any member of the insect family Tabanidae (order Diptera), but more specifically any member of the genusTabanus. These stout flies, as small as a housefly or as large as a bumble bee, are sometimes known as greenheaded monsters; their metallic or iridescent eyes meet dorsally in the male and are separate in the female. Gad fly, a nickname, may refer either to the fly’s roving habits or to its mouthparts, which resemble a wedge-shaped miner’s tool. Other such names are breeze fly and ear fly. One of the most common species (Tabanus lineola) has bright-green eyes and is known as green head. The genus Chrysops, usually known as deer fly, is slightly smaller than Tabanus and has dark markings on the wings.

Horse fly (Tabanus).

Dennis Ray

Horse fly (Tabanus trimaculatus).

Fran Hall—The National Audubon Society Collection/Photo Researchers

Adult horse flies are fast, strong fliers that are usually found around streams, marshes, and wooded areas. They may be carriers of various animal diseases such as anthrax, tularemia, and trypanosomiasis. Females deposit long, flat, black eggs in clusters; the eggs are laid on the grass. Horse flies overwinter in the larval stage, pupate in spring, and emerge as adults by late June.

The bloodsucking females may be serious pests to humans and animals. When abundant they sometimes suck three ounces or more of blood a day from a host. When the genus Haematopota becomes too abundant, agricultural work can only be done at night when the fly is not active. The males feed on nectar, honeydew, and plant sap.

Control methods include draining or oiling of breeding places; sprays are not usually successful. Covering a horse with a blanket or fly net helps protect it from horse fly attacks.

Some scientists maintain that the zebra’s stripes evolved to thwart horsefly infestation, which would have reduced the chance for disease. Indeed, there is evidence that the zebra’s stripes disrupt the horizontal pattern of polarized light reflected from dark surfaces that normally attracts horseflies. This would make the zebra’s striped fur less attractive to horseflies than the solid-coloured...

Horse bot flies (subfamily Gasterophilinae) include species of Gasterophilus, a serious horse pest. The adult horse fly, often known as a gad fly, deposits between about 400 and 500 eggs (nits) on the horse’s forelegs, nose, lips, and body. The larvae remain in the eggs until the horse licks itself. With the stimulus of moisture and friction, the larvae emerge and are ingested. They...